In recent years, computer systems have exhibited major advances in speed and in miniaturization at significant reduction in cost. Concurrent with these advances have been major efforts to develop information storage and retrieval systems which are also low cost and still compatible with the high speeds with which these new systems operate. Much of this effort has been directed toward the development of optical disc memory devices because of their rapid write time (&gt;1 M bit per second), rapid access time (&lt;0.1 sec.), high density (&gt;10.sup.8 bits per sq. in.), and low cost (.about.10.sup.-4 cents per bit). (See Kenney, et al., IEEE Spectrum, pages 33-38, February 1979). To date, numerous types of materials have been developed for optical discs; however, most of these have the disadvantage of storing information permanently; i.e., they cannot be erased or edited. (See R. A. Bartolini, et al., IEEE Spectrum, pages 20-28, August 1978)
Materials which are exceptions to this general rule appear to fall into the following classifications: certain thermoplastics, photochromics, chalcogenides, magnetooptical materials, photoferroelectric materials, photoconductive/electrooptical materials and electrooptic materials. However, each of these has significant disadvantages. For example, thermoplastics such as polyvinylcarbozole/polystyrene require a pre-exposure corona charge. In addition, they use surface relief storage which has a relatively low contrast ratio, a long erase time, difficulties in local erase, and a short lifetime (.about.100 cycles). Photochromics such as spiropyrane typically require blue or ultra-violet light for write and/or erase and hence are not compatible with present semiconductor lasers. Furthermore, most photochromics are subject to fatigue, which severely limits cycle lifetime, and the stored data tends to fade in just a few minutes. Chalcogenides such as TeAsGe typically exhibit a low cycle lifetime (.about.cycles) and have a relatively low contrast ratio for erasable media. Magnetooptical materials such as the rate earth iron garnets usually require an external magnetic field. Materials with the largest magnetooptical effects (garnets) require micro-patterning to get high density storage and have high optical insertion loss. Photoferroelectric materials such as Bi.sub.4 Ti.sub.3 O.sub.12 require single crystals which are difficult to prepare in large areas. Large area photoferroelectrics can be fabricated as ceramic materials but the ceramics are subject to fatigue. Photoconductive/electrooptic materials such as Bi.sub.12 SiO.sub.20 have limited data storage times, on the order of several hours, and also require large single crystals. Electrooptic materials such as LiNbO.sub.3 also require single crystals and stored information is erased during readout unless the image is thermally fixed whence another thermal treatment is required for erasure.
The use of liquid crystal materials, particularly smectic liquid crystals is well known in the prior art for certain display devices, and stationary memory systems have been developed which use this media for information storage. (See U.S. Pat. No. 3,796,999 entitled LOCALLY ERASABLE THERMO-OPTIC SMECTIC LIQUID CRYSTAL STORAGE DISPLAYS; and Dewey, et al., SID 77 Digest, 108 (1977).) However, the prior art does not indicate the development of any liquid crystal devices which can function under large accelerations to provide rapid access to data (in a computer disc application, a liquid crystal could experience accelerations exceeding 30,000 m/sec.sup.2 depending on the disc size and the desired data rate). The obvious flow-related problems associated with liquids in general suggests that liquid crystals could not be used in optical disc-type memory systems, where large rotational velocities are necessary to achieve the desired data input-output rates. Indeed, rotational velocities exceeding 10,000 rpm may be desirable for some applications.
Furthermore, one approach to information storage in smectic liquid crystals is to create radiation scattering defects in the liquid crystal medium. Such an approach, in light of the specific physical characteristics of defects in these crystals would seem to indicate that in a rotating system, smectic liquid crystals would have severe problems with information stability. For example, it is expected that the cores of defects in smectic liquid crystals structurally approach the isotropic phase and hence have a measurably lower density than the smectic phase (.about.0.1%, see S. Torza and P. E. Cladis Phys. Rev. Letters, 32(25), 1406 (1974). It would therefore be reasonable to expect significant defect motion or even destruction of the defect (information) pattern in the liquid crystal as a result of centrifugal forces developed at high angular velocities. It would also be reasonable to expect fluid motion to occur during the startup and slowdown phases, further destroying any information-containing defect systems.
Other studies on the structure of defects in homeotropically aligned smectic layers have shown that the information-carrying defects are focal conics which form polygonal arrays spanning the liquid crystal cell (see C. S. Rosenblatt, R. Pindak, N. A. Clark, R. B. Meyer, J. Physique 38(9), 1105(1977)); such arrays could be expected to be quite susceptible to any fluid motion. Furthermore, other literature has shown that cell dilatations of the order of 100 A are sufficient to cause spontaneous formation of polygonal focal conic defect arrays. (See N. A. Clark, Phys. Rev., A14, 1551 (1976).) With such extreme sensitivity to dilatations, the formation of such polygonal focal conic arrays could be expected to develop at high angular velocities, thereby destroying any information pattern contained in the liquid crystal medium.
For all of these reasons, the use of liquid crystal materials in a rotating data storage system has not heretofore been suggested in the art.